1. Field of the Invention
The present invention relates to a coupling structure between a screw shaft for driving an injection screw driving body and a motor shaft of electric motor in a motor-driven injection device, in which the screw shaft and the motor shaft are coupled directly to each other via splines.
2. Description of the Related Art
In addition to a system for indirectly coupling an electric motor to a screw shaft via a pulley and a timing belt, a system for directly coupling a screw shaft to a motor shaft of an electric motor is included in a motor-driven injection device, in which a screw shaft is screwed with a nut member provided on the injection screw driving body guided in a linear direction, and a rotating movement by an electric motor (i.e., a servo motor), transmitted to the screw shaft by the nut member is converted into a linear movement, so that the injection screw driving body is allowed to advance together with a screw disposed inside of a heating cylinder, thereby injecting a molten resin.
Such a directly coupling system includes a method, in which the rear end of a screw shaft is fitted into a recess formed at a motor shaft end, thereby coupling them via a parallel key, or coupling them in mutual engagement via splines, so as to transmit rotational torque to the screw shaft.
The coupling via the parallel key requires a clearance for facilitating the fitting and maintenance of the screw shaft. Therefore, the clearance is enlarged by abruptly forward or reverse rotation of an electric motor, thereby causing seizure due to fretting at a shaft portion. This raises a problem that the motor shaft cannot be separated from the screw shaft at the time of maintenance or inspection work, which are thus inevitably omitted.
In contrast, the coupling via the splines does not require particularly any clearance as described above, and further, can prevent abrasion or seizure at a portion in mutual engagement of the splines with the application of a lubricant, thereby solving the problem experienced in the case of the parallel key. However, the spline for the motor shaft is difficult to be machined, which causes high cost. Therefore, it is difficult to adopt an involute spline, which is suitable for mutually coupling rotary shafts in other fields.
Such an involute spline has a cross-sectional shape of an involute spur gear, and has centripetal characteristics, which intends to automatically become coaxial at a engaging portion, thereby preventing any inconvenience caused by center runout. Thus, a normal engaging state can be held for a long period. Furthermore, the involute spline is preferable from the viewpoints of coupling strength, assembly or disassembly, maintenance, inspection work and the like.